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Date: Thu, 31 Mar 2022 16:58:00 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
baolin.wang@...ux.alibaba.com
Cc: Jagdish Gediya <jvgediya@...ux.ibm.com>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
dave.hansen@...ux.intel.com, Fan Du <fan.du@...el.com>
Subject: Re: [PATCH] mm: migrate: set demotion targets differently
"Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com> writes:
> "Huang, Ying" <ying.huang@...el.com> writes:
>
>> "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com> writes:
>>
>>> "Huang, Ying" <ying.huang@...el.com> writes:
>>>
>>>> "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com> writes:
>>>>
>>>>> "Huang, Ying" <ying.huang@...el.com> writes:
>>>>>
>>>>>> Hi, Jagdish,
>>>>>>
>>>>>> Jagdish Gediya <jvgediya@...ux.ibm.com> writes:
>>>>>>
>>>>>
>>>>> ...
>>>>>
>>>>>>> e.g. with below NUMA topology, where node 0 & 1 are
>>>>>>> cpu + dram nodes, node 2 & 3 are equally slower memory
>>>>>>> only nodes, and node 4 is slowest memory only node,
>>>>>>>
>>>>>>> available: 5 nodes (0-4)
>>>>>>> node 0 cpus: 0 1
>>>>>>> node 0 size: n MB
>>>>>>> node 0 free: n MB
>>>>>>> node 1 cpus: 2 3
>>>>>>> node 1 size: n MB
>>>>>>> node 1 free: n MB
>>>>>>> node 2 cpus:
>>>>>>> node 2 size: n MB
>>>>>>> node 2 free: n MB
>>>>>>> node 3 cpus:
>>>>>>> node 3 size: n MB
>>>>>>> node 3 free: n MB
>>>>>>> node 4 cpus:
>>>>>>> node 4 size: n MB
>>>>>>> node 4 free: n MB
>>>>>>> node distances:
>>>>>>> node 0 1 2 3 4
>>>>>>> 0: 10 20 40 40 80
>>>>>>> 1: 20 10 40 40 80
>>>>>>> 2: 40 40 10 40 80
>>>>>>> 3: 40 40 40 10 80
>>>>>>> 4: 80 80 80 80 10
>>>>>>>
>>>>>>> The existing implementation gives below demotion targets,
>>>>>>>
>>>>>>> node demotion_target
>>>>>>> 0 3, 2
>>>>>>> 1 4
>>>>>>> 2 X
>>>>>>> 3 X
>>>>>>> 4 X
>>>>>>>
>>>>>>> With this patch applied, below are the demotion targets,
>>>>>>>
>>>>>>> node demotion_target
>>>>>>> 0 3, 2
>>>>>>> 1 3, 2
>>>>>>> 2 3
>>>>>>> 3 4
>>>>>>> 4 X
>>>>>>
>>>>>> For such machine, I think the perfect demotion order is,
>>>>>>
>>>>>> node demotion_target
>>>>>> 0 2, 3
>>>>>> 1 2, 3
>>>>>> 2 4
>>>>>> 3 4
>>>>>> 4 X
>>>>>
>>>>> I guess the "equally slow nodes" is a confusing definition here. Now if the
>>>>> system consists of 2 1GB equally slow memory and the firmware doesn't want to
>>>>> differentiate between them, firmware can present a single NUMA node
>>>>> with 2GB capacity? The fact that we are finding two NUMA nodes is a hint
>>>>> that there is some difference between these two memory devices. This is
>>>>> also captured by the fact that the distance between 2 and 3 is 40 and not 10.
>>>>
>>>> Do you have more information about this?
>>>
>>> Not sure I follow the question there. I was checking shouldn't firmware
>>> do a single NUMA node if two memory devices are of the same type? How will
>>> optane present such a config? Both the DIMMs will have the same
>>> proximity domain value and hence dax kmem will add them to the same NUMA
>>> node?
>>
>> Sorry for confusing. I just wanted to check whether you have more
>> information about the machine configuration above. The machines in my
>> hand have no complex NUMA topology as in the patch description.
>
>
> Even with simple topologies like below
>
> available: 3 nodes (0-2)
> node 0 cpus: 0 1
> node 0 size: 4046 MB
> node 0 free: 3478 MB
> node 1 cpus: 2 3
> node 1 size: 4090 MB
> node 1 free: 3430 MB
> node 2 cpus:
> node 2 size: 4074 MB
> node 2 free: 4037 MB
> node distances:
> node 0 1 2
> 0: 10 20 40
> 1: 20 10 40
> 2: 40 40 10
>
> With current code we get demotion targets assigned as below
>
> [ 0.337307] Demotion nodes for Node 0: 2
> [ 0.337351] Demotion nodes for Node 1:
> [ 0.337380] Demotion nodes for Node 2:
>
> I guess we should fix that to be below?
>
> [ 0.344554] Demotion nodes for Node 0: 2
> [ 0.344605] Demotion nodes for Node 1: 2
> [ 0.344638] Demotion nodes for Node 2:
If the cross-socket link has enough bandwidth to accommodate the PMEM
throughput, the new one is better. If it hasn't, the old one may be
better. So, I think we need some kind of user space overridden support
here. Right?
> Most of the tests we are doing are using Qemu to simulate this. We
> started looking at this to avoid using demotion completely when slow
> memory is not present. ie, we should have a different way to identify
> demotion targets other than node_states[N_MEMORY]. Virtualized platforms
> can have configs with memory only NUMA nodes with DRAM and we don't
> want to consider those as demotion targets.
Even if the demotion targets are set for some node, the demotion will
not work before enabling demotion via sysfs
(/sys/kernel/mm/numa/demotion_enabled). So for system without slow
memory, just don't enable demotion.
> While we are at it can you let us know how topology will look on a
> system with two optane DIMMs? Do both appear with the same
> target_node?
In my test system, multiple optane DIMMs in one socket will be
represented as one NUMA node.
I remember Baolin has different configuration.
Hi, Baolin, Can you provide some information about this?
>>
>>> If you are suggesting that firmware doesn't do that, then I agree with you
>>> that a demotion target like the below is good.
>>>
>>> node demotion_target
>>> 0 2, 3
>>> 1 2, 3
>>> 2 4
>>> 3 4
>>> 4 X
>>>
>>> We can also achieve that with a smiple change as below.
>>
>> Glad to see the demotion order can be implemented in a simple way.
>>
>> My concern is that is it necessary to do this? If there are real
>> machines with the NUMA topology, then I think it's good to add the
>> support. But if not, why do we make the code complex unnecessarily?
>>
>> I don't have these kind of machines, do you have and will have?
>>
>
>
> Based on the above, we still need to get the simpler fix merged right?
Or user overridden support?
Best Regards,
Huang, Ying
[snip]
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